1. Field of the Invention
The present invention relates to the intraoperative and postoperative monitoring of cardiac surgery patients by establishing and monitoring electrophysiologic parameters to determine myocardial ischemic injury during and after cardiac surgical procedures.
2. Description of the Prior Art
The current sophisticated techniques which have led directly to improved care and treatment of patients during and after cardiac surgery have been provided without a reliable technique being available to monitor the viability and integrity directly of the myocardium either during or after cardiac surgery.
Myocardial ischemic injury may occur at any time during perioperataive periods in patients who are undergoing cardiac surgical procedures. This period of time when surgery is ongoing and when treatment is ongoing after surgery, is filled with the dangers of hypotension secondary to the general anesthetic agent employed. The intraoperative period is characterized by intensive efforts to maintain a satisfactory relationship between myocardial supply and myocardial demand during cardiopulmonary bypass using a wide variety of techniques for myocardial preservation. During the postoperative period, attention is primarily directed towards maintaining an adequate cardiac output by employing various types of pharmacologic and mechanical therapeutic support.
The methods currently used to determine the adequacy of the myocardial preservation during cardiopulmonary bypass are capable of detecting myocardial injury only after it is too late to reverse the injury. Likewise, although it is possible to monitor the status of the lungs, brain, kidneys, blood components, and coagulation factors during and after cardiac surgery, no reliable technique is available to assess myocardial integrity. Therefore, it has been the experience of most cardiac surgeons who have performed a cardiopulmonary bypass which has been smoothly initiated and carried through to have experienced instances during which an apparently uncomplicated technical operation with "good myocardial preservation" has been followed by an inability of the myocardium to generate a contractile force which is adequate to allow termination of cardiopulmonary bypass.
With regard to the area of anesthetics, a similar situation has arisen to that of the mycardial integrity observed concerning a normal bypass. That is, despite statistically acceptable operative mortality figures, electrocardiographic changes and serum isoenzyme levels consistent with intraoperative myocardial infarction may be present in as many as 25% of patients undergoing cardiac surgery. Thus far the major effort at preventing perioperative myocardial injury has centered on the intraoperative period. This goal of myocardial preservation has currently been based upon a simple observation that if the myocardium can be maintained in its pre-bypass state during the period of cardiopulmonary bypass, it should maintain that prebypass state after cessation of cardiopulmonary bypass. Intermittent aortic cross-clamping, selective coronary perfusion when associated with valvular surgery, induced ventricular fibrillation, hypothermia, and potassium cardioplegia have been previously employed to "preserve" the myocardium during the cardiopulmonary bypass.
Concerning the improvements which have recently been made to postoperative pharmacological methods and treatments such as nitroprusside and Dopamine, it is to be noted that while they have made control of cardiac output much easier during the first critical hours and days following cardiac surgery, they do not leave the domain of emperical studies because the immediate effect of the various drugs on myocardial integrity is unknown.
Another area of concern with regard to myocardial integrity exists during the cardiopulmonary surgery when an electrocardioplegic arrest has been effected. The concept of elective cardioplegic arrest for myocardial preservation involves the electrical and mechanical arrest of the heart through the use of various clamping techniques and the infusion of a cardioplegic solution. Continuous monitoring of ventricular septal temperature has been used to improve the surgeon's ability to determine the optimal time to re-infuse cardioplegic solution when the period of cardioplegic arrest is prolonged. Electrical arrest is commonly documented by continuous monitoring of a peripheral limb-lead electrocardiogram and the mechanical arrest is verified by visual inspection. Aside from these two observations with regard to monitoring of the activity of the heart the only other assurance given to a surgeon of the complete arrest of the heart has been the re-perfusion with the cardioplegic solution at predetermined time periods. These predetermined time periods are based on empirical observations with regard to the length of time a normal heart is "quiet" after the initial infusion of the solution. There is no other, more accurate, determination with regard to the assurance of the complete inactivity of the heart during the progress of the operation. Because the energy requirements of the myocardium are greater when any activity is occurring, the previous methods of determining electrical and mechanical arrest lack a sufficient observable basis for concluding inactivity in the heart with respect to preserving myocardial integrity.
Despite all of these recent advances in myocardial preservation, the emphasis for improvement has been markedly fixed on determining which technique is the best or most effective. It has been demonstrated that any of the above-mentioned techniques is adequate in a particular patient and that none of the techniques is adequate in all of the patients. This leaves the field with a critical need for a means to determine during cardiopulmonary bypass the exact point in time when myocardial preservation becomes inadequate, regardless of the technique employed. Standard electrocardiographic changes, serum isoenzyme changes, and postoperative cardiac function analysis are capable of indicating the adequacy of intraoperative myocardial preservation only in a retrospective manner. Likewise, as indicated above, potential harmful effects of postoperative pharmacological agents on the myocardium of a given patient can be detected only after these effects have occurred.